Microelectronic devices contain millions of electrical circuit components, mainly transistors assembled in integrated circuit (IC) chips, but also resistors, capacitors, and other components. The integrated circuit component may comprise a single bare chip, a single encapsulated chip, or an encapsulated package of single or multi-chips. These electronic components are interconnected to form the circuits, and eventually are connected to and supported on a carrier or substrate, such as a printed wire board.
The various materials used to manufacture the integrated circuits and their related interconnect materials are susceptible to environmental, moisture, and mechanical damage. Protection is provided by encapsulation of the electronic component within a polymeric material. Encapsulation can be performed by a transfer molding process in which the component is loaded into a mold cavity, constrained, and the polymeric encapsulant transferred from a reservoir into the cavity under pressure. Typically, the encapsulant is a thermosetting polymer, which then cross-links and cures to form the final assembly. Encapsulation also can be performed by dispensing an aliquot of polymeric encapsulant onto the component, such as a chip or integrated circuit supported on a substrate, and subsequently curing the composition.
For most commercial and industrial end uses, particularly those utilizing chip-on-board packages and multi-chip modules, the encapsulation is accomplished with polymeric thermosetting materials. The preferred thermosetting package encapsulation materials must have a viscosity and a thixotropic index that allows easy dispensability by syringe, sufficient adhesion to the components, low ionic content to avoid corrosion of the metallization, adequate mechanical strength, high thermal and moisture resistance at application temperatures, and matched coefficient of thermal expansion to the materials it contacts.
For single chip packaging involving high volume commodity products, a failed chip can be discarded without significant loss. However, it becomes expensive to discard multi-chip packages with only one failed chip, and the ability to rework the failed component would be a manufacturing advantage. Today, one of the primary thrusts within the semiconductor industry is to develop not only a package encapsulant that will meet all the requirements for protection of the component, but also a package encapsulant that will be reworkable, allowing for the failed component to be removed without destroying the substrate.
In order to achieve the required mechanical performance and reworkability, relatively high molecular weight thermoplastics would be the preferred compositions for package materials. These materials, however, have high viscosity or even solid film form, which are drawbacks to the manufacturing process. Therefore, there is a need for new encapsulant compositions that are easily dispensable to conform with automated manufacturing processes, and that are reworkable.